The sea contains most of the earth's water. Water originally started
to form on earth when the young, hot planet's surface cooled below the
boiling point of water. Once the surface was cool enough for water to
stand, volcanoes over millions and millions of years continued to add
water to the surface. It arrived in volcanic gases, which are mostly
steam, and then condensed to help form the sea.

After a long time, the amount of water in the sea stopped increasing
because less 'new' water was coming up out of the volcanoes. There
are still a lot of volcanoes producing steam, but most of this water
is recycled. This happens because some water from the seas gets
trapped in sediment on the seafloor where it is carried down into the
earth at 'subduction zones'. Most of the volcanoes we see today, like
Ruapehu or White Island, are found with these subduction zones. The
subduction zone for Ruapehu and White Island is under the Hikurangi
trench, along the east coast of the North Island.

The 'saltiness' of seawater isn't from volcanoes directly. Salts in
rocks and soils on land are slowly dissolved (chemically broken down)
by water (rainwater, river water etc). Rivers then carry these salts
to the oceans. There is little salt dissolved in river water so they
don't seem salty to us. The ocean is salty mostly because, over
millions and millions of years, rivers have been carrying into it
dissolved salts from the chemical breakdown of rocks and soils on
land.

Once the water is in the ocean, some of it evaporates to form
clouds, leaving the salt behind in the water. The clouds drift over
land, making rain that feeds the rivers, carrying yet more salt to the
ocean. The amount of salt in the ocean stays fairly constant even
though the rivers continue to bring in salt because it is in chemical
balance. If salt levels get too high, salt crystals form and separate
from the water, taking away the excess salt.